Columnar basalt
When basaltic lava flows cool and solidify they often develop fractures perpendicular to their surface because the lava's volume decreases. Individal columns tend to have a hexagonal (six-sided) cross section. They may become exposed when the lava flow gets eroded from the side.
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Columnar basalt at Svartifoss (Skaftafell), Iceland. Photo: J. Alean |